1. Field of the Invention
The present invention relates to an optical apparatus having a visual axis detection means, and more particularly, to an optical apparatus, such as a camera, having a visual axis detection means designed to detect an axis of a focal point at which an observer (a photographer) is observing, i.e., a visual axis (visual line), on an observing surface (a focusing surface) on which an image of a subject is formed by a photographing system by utilizing an image reflected by an eyeball which is obtained by illuminating the surface of the eyeball of the observer.
2. Related Background Art
A variety of visual axis (visual line) detecting devices for detecting the point at which an observer is observing, i.e., a visual axis (visual line), on an observing surface have been proposed.
In the visual axis detecting device disclosed in, for example, Japanese Patent Laid-Open No. 61-172552, parallel rays of light from a light source are projected on the front portion of an eyeball of the observer, and the visual line is obtained by utilizing an image reflected by a cornea, which is formed by the light reflected by the cornea, as well as the position at which the pupil is formed. FIG. 8A is a schematic view of a visual axis detecting optical system, and FIG. 8B shows distribution of the level of a signal output from a photoelectric element array 6.
In the visual axis detection system shown in FIG. 8A, a light source 5, such as a light-emitting diode, for emitting an infrared radiation invisible to an observer is disposed on the focal plane of a projection lens 3.
The infrared radiation emitted from the light source 5 passes through the projection lens 3 which creates a beam made up of parallel rays of light. The radiation is reflected by a half mirror 2, and then it irradiates a cornea 21 of an eyeball 201. An image d reflected by the cornea 21, which is formed by part of the infrared radiation reflected by the surface of the cornea 21, passes through the half mirror 2 and is directed to a position Zd' on the photoelectric element array 6 by a light-receiving lens 4 to form an image thereon.
Rays of light from end portions a and b of an iris 23 pass through the half mirror 2 then the light-receiving lens 4 and form images of the end portions a and b at positions Za' and Zb' on the photoelectric element array 6. When a rotational angle .theta. of an optical axis N of the eyeball is small with respect to the optical axis M of the light-receiving lens 4, the Z coordinate Zc of the central position c of a pupil 24 is expressed as EQU Zc.perspectiveto.(Za+Zb)/2
where Za and Zb are respectively Z coordinates of the end portions a and b of the iris 23.
The rotational angle .theta. of the optical axis N of the eyeball substantially satisfies the relation expressed by the following equation EQU OC * SIN .theta..perspectiveto.Zc-Zd (1)
where Zd is the Z coordinate of the position d at which the image reflected by the cornea is generated and OC is the distance between the center of curvature O of the cornea 21 and the center C of the pupil 24. Hence, an operation means 9 can obtain the rotational angle .theta. of the optical axis N of the eyeball 201 by detecting the positions of the characteristic points projected on the photoelectric element array 6 (the image d reflected by the cornea and the end portions a and b of the iris), as shown in FIG. 8B. At that time, equation (1) is transformed into ##EQU1## where .beta. is the magnification determined by a distance L1 between the position d at which the image reflected by the cornea is generated and the light-receiving lens 4 and a distance L0 between the light-receiving lens 4 and the photoelectric element array 6, which magnification is, in general, substantially constant.
The optical axis N of the eyeball of the observer does not coincide with the visual axis. Ser. No. 671,656 discloses the structure for detecting the visual axis corrected by the angle between the optical axis of the eyeball of the observer and the visual axis. In this structure, after the rotational angle .theta. of the optical axis of the eyeball of the observer in the horizontal direction is calculated, it is corrected by the angle .delta. between the optical axis of the eyeball and the visual axis, and the visual axis .theta.H of the observer in the horizontal direction is expressed as EQU .theta.H=.theta..+-..delta. (3)
In the above equation, assuming that the rightward rotation with respect to the observer is positive, if the observer looks into the observing device with his or her left eye, the sign + is used. If the observer looks with his or her right eye, the sign - is selected.
In the example shown in FIG. 8A, the eyeball of the observer is rotated on a Z-X plane (e.g., on the horizontal plane). However, rotation of the eyeball of the observer on the X-Y plane (e.g., on the vertical plane) can also be detected in a similar manner. In this case, since the vertical component of the visual axis of the observer coincides with the vertical component .theta.' of the optical axis of the eyeball, the visual axis .theta.V in the vertical direction is expressed as EQU .theta.V=.theta.' (4)
FIG. 9 shows the layout of an optical system in which the visual axis detection device shown in FIG. 8A is applied to part of the finder system of a single-lens reflex camera.
In the structure shown in FIG. 9, a light from a subject passes through a photographic lens 101 and is reflected by a quick return mirror 102. The reflected light forms an image in the vicinity of the focal plane of a focusing screen 104. The light from the subject diffused by the focusing screen 104 passes through a condenser lens 105, a pentagonal roof prism 106 and then an eye-piece lens 1 having a light dividing surface la, and is directed to an eye point 201a of a photographer.
A visual axis detection optical system includes an illumination means (having an optical axis S) having the light source 5, such as a light-emitting diode, for emitting infrared radiation invisible to the eyes of the photographer (observer), and the projection lens 3, and a light-receiving means (having an optical axis t) having the photoelectric element array 6, the half mirror 2 and the light-receiving lens 4. The visual axis detection optical system is disposed above the eye-piece lens 1 having the light dividing surface 1a, which comprises a dichroic mirror. The infrared radiation emitted from the infrared radiation emitting diode 5 is reflected by the light dividing surface 1a and then irradiates the eyeball 201 of the photographer. Part of the infrared radiation reflected by the eyeball 201 is reflected by the light-dividing surface 1a again, passes through the light-receiving lens 4 and the half mirror 2 and enters the photoelectric element array 6. From the image data for the eyeball obtained on the photoelectric element array 6 (for example, an output signal shown in FIG. 7B), the operation means 9 calculates the direction of the visual axis of the photographer, that is, the operation means 9 obtains the point (focal point) on the focusing screen 104 at which the observer is looking.
From the horizontal visual axis .theta.H and the vertical axis .theta.V, the point (Zn, Yn) on the focusing screen 104 at which the photographer is looking is expressed by EQU Zn.perspectiveto.m * .theta.H EQU Yn.perspectiveto.m * .theta.V (5)
where m is the constant determined by the finder system of a camera.
In a single-lens reflex cameras with an auto focus detecting device having a plurality of focus detectable points on the screen including the central point thereof which can be selected by the photographer as a point at which auto focusing is detected, when the point on the focusing screen 104 at which the photographer is observing is determined, the operation of selecting one of the plurality of points at which focusing is automatically detected is be omitted. The determined point is regarded as the point at which focusing is to be detected, and automatic focus detection is conducted at that selected point.
Cameras are operated by photographers who wear glasses or who do not wear glasses. In general, the eye point of the photographer who does not wear eye glasses is separated from that of the photographer who do not wear eye glasses by several millimeters.
In the visual axis detecting method shown in FIG. 8, the eyeball of the observer who is located at a predetermined position is illuminated, the light reflected by the eyeball is received by the photoelectric element array, and the visual axis is detected on the basis of the obtained eyeball image. Hence, when the eye point of the observer is separated greatly from the predetermined position, the detected eyeball image deteriorates, thus reducing the detection accuracy of the visual axis.
In order to overcome this problem, the present applicants filed Patent Application No. hei 1-247332 which discloses the camera having the visual axis detection device capable of detecting the visual axis with a high degree of accuracy by controlling the visual axis detection method using data representing whether or not the photographer wears the eye glasses which is input by means of a switch provided on part of the camera. U.S. Pat. No. 4,575,314 discloses the television camera having the visual axis detecting device.